Human cytomegalovirus has been linked to numerous types of human malignant diseases such as neuroblastoma, prostate cancer, cervical cancer and colon carcinoma. The virus is reactivated in AIDS and in transplant patients. Its transforming potential is of great concern in vaccine development. We reported two transforming domains in the long unique segment of human cytomegalovirus (HCMV) genome: i) mtrII (980 bp) and ii) mtrIII (7.5 kb). Transforming functions were studied by generating deletion clones, transfecting the clones by DNA mediated gene transfer into mouse 3T3 or Rat-2 cell lines and assaying the cells for anchorage independent growth in agarose and for tumorigenicity in mice. The mtrII region was found to contain 3 open reading frames (ORFs) (79, 83 and 34 aa) by DNA sequence analysis. We also reported the promoter activity in the upstream region of ORFs in mtrII using CAT assays and the detection of mtrII transcripts in the transformants. Similar mRNA species were also detected in HCMV infected cells. Along these lines, mtrII transforming activity was also tested in human cells. We recently identified that mtrII was capable of transforming human epidermal keratinocyte cell line (RHEK-1) to tumorigenicity, and the tumors were diagnosed as poorly differentiated carcinoma. Further experiments will be necessary to identify the role of the ORFs in transformation by generation of further deletion clones or mutagenesis of the ORFs. Also mapping of the transcripts will be attempted to identify whether the same gene is expressed in both the transformed and the infected cells. Regarding mtrIII (7.5 kb) transforming domain, the CMV major immediate early gene (IE-1) is contained within it. This IE-1 gene is a candidate subunit vaccine. Our deletion analysis localized the transforming activity of mtrIII to a 2.1 kb region beyond IE-1 gene. This region also contained several ORFs which are currently under investigation for their role in transformation. These studies will be applicable to generate a safe live attenuated CMV vaccine, a subunit vaccine and most importantly to use CMV as a cloning vehicle for gene therapy.